On the first personal computers (PCs) developed by IBM, the operating system and applications executed were relatively compact, sharing about one megabyte of address space. PCs including desktop and portable laptop computers have subsequently evolved to much more sophisticated designs. Graphic operating systems and applications requiring much more memory have become common, and it is not unusual for PCs and laptop computers to be sold with 16 or more megabytes of random access memory (RAM) installed. In contrast to the increasing amount of RAM installed on PCs, portable personal information (PIM) devices have recently been introduced that include a microcomputer or microprocessor provided with only enough RAM to enable the dedicated PIM functions to be performed. PIM devices must be distinguished from personal, data assistants (PDAs) that include sufficient RAM to enable small application programs specifically developed for the PDA to be executed. PIM devices provide very limited computational functions, and much of the functionality of the devices is programmed into read only memory (ROM). RAM is typically not provided on a PIM device for running application programs that are downloaded into non-volatile read/write memory. An example of a PIM device is the Timex DATA LINK.TM. wristwatch, which provides timekeeping functions and stores PIM data that are downloaded from a PC through a sensor that responds to a predefined modulation of scan lines on the PC monitor.
Data can be downloaded into a non-volatile read/write memory of a portable computing device such as a lap top computer from a conventional PC using a variety of different mediums, including: radio frequency (RF) links, infrared data links, and data cable connections. In addition to the screen modulation technique noted above, these same data transfer mediums can be employed to download data into a portable PIM device. A PC is preferably used for compiling/entering the data, since the keyboard of the PC permits more efficient data entry, the hard drive of a PC conveniently provides long term storage, and data are more easily reviewed on the PC display. Application programs are not normally downloaded to a PIM device from a PC, because the 2-4 Kbytes (or less) of RAM provided on the typical PIM device is generally considered too small for executing downloaded code.
More recently, however, a combination pager and PIM device has been developed that will receive conventional paging messages and paging objects that are used to update PIM data and other data maintained in non-volatile read/write memory in the device. The data stored in the device includes the typical PIM type of data noted above, e.g., addresses, telephone numbers, and names. However, the design specification for this dual purpose paging device requires that it also be capable of executing small application programs called "applets" using the very limited amounts of RAM provided on the device, i.e., typically less than 4 Kbytes. Those familiar with programming applications for execution by a microprocessor will appreciate that with even the most careful and efficient coding, it is generally not possible to implement more than a trivial application with machine instructions that will fit into 4 Kbytes of RAM. Accordingly, a new approach is required that will enable processing of larger amounts of application code in a very limited amount of RAM.
Another facet of the problem is that the machine instructions comprising an applet must be written in very efficient format that is optimized for the limited memory environment in which the applet will be executed. Processing programming language instructions with a software interpreter has been used on PCs in the prior art to manage the size of application programs. An example of an interpreter for use in a limited processing environment is the BASIC STAMP.TM. circuit, which is a single integrated circuit that includes an embedded BASIC language interpreter. However, the use of an integrated circuit to facilitate an interpreter is not particularly useful in the present case, because it would increase the cost of the device to an unacceptable level. Also, the interpreter circuit produces machine language instructions, and too much RAM would be required to store and run the machine language instructions produced by the interpreter circuit. Instead, the microcomputer should be able to execute applications written in a different form that is tuned to the limited processing environment of the device. The code should include short instructions that represent more complex computations and algorithms, thereby keeping the size of the applets extremely small and within the constraints of the memory and capabilities of the processing environment. Further the code loaded into RAM when an applet is executed should preferably be processor independent.